The objective of these studies is to understand the early events in the regulation of alternative pre-mRNA splicing, particularly as it relates to the splicing of GH pre-mRNA. The specific aims to be addressed are: 1. The molecular details of the initial enhancer recognition event will be determined, including the precise nucleotide sequence of the bGH exonic enhancer, and the role of important functional domains of SF2/ASF (RNA binding and Arg/Ser domains) will be defined. 2. The mechanism whereby the exon specific enhancer (ESE)-splicing factor complex proceeds to activate suboptimal splice sites will be defined by determining the additional splicing components that are recruited to the enhancer complex, the contribution of protein-protein interactions to this recruitment, and the role of the enhancer in bringing distant sites into the complex. 3. The GH model will be used to determine the role of changing splicing factor concentrations in developmentally-regulated alternative patterns of splicing.